1. Field of the Invention
The invention relates to platinum catalysts supported on nanosize titanium dioxide (Pt/TiO2 catalysts), their use in hydrosilylation, a hydrosilylation process using such catalysts and compositions comprising such catalysts.
2. Description of the Related Art
The addition of Si—H-functional compounds onto compounds having aliphatic unsaturated bonds, in particular C═C double bonds, (hydrosilylation) has been known for a long time.
Si-containing organic compounds, organosilanes and organopolysiloxanes can be prepared by hydrosilylation. Hydrosilylation has been used in particular in the addition-crosslinking curing of organopolysiloxanes in the silicone industry, for example for producing elastomers, molding compositions in the dental industry or antiadhesive coatings in the paper and films industry.
Catalysts used for the hydrosilylation reaction are most frequently platinum and its compounds, with the platinum being used either in metallic form, as metal immobilized on an inorganic support, as platinum salts, or in the form of a soluble or insoluble platinum complex.
Up to the present day, the major part of hydrosilylation reactions carried out in industry are carried out using the “Karstedt catalyst” known from U.S. Pat. No. 3,715,334 and U.S. Pat. No. 3,775,452, a dimeric platinum-tetramethyldivinyldisiloxane complex which can be described by the formula [Pt2(TMDVS)3] (TMDVS=tetramethyldivinyldisiloxane). The Karstedt catalyst is prepared from hexachloroplatinic acid H2PtCl6, which is likewise frequently used as hydrosilylation catalyst.
Platinum-catalyzed hydrosilylation reactions in which activation is by means of UV and/or visible light are also known from the prior art. Known photoactive platinum compounds are, for example, CpPtMe3 known from U.S. Pat. No. 4,510,094 and WO92/10543, Pt(acac)2 known from EP 0398701, (COD)PtR2, where R is an aryl radical, known from U.S. Pat. No. 4,530,879, WO92/10529 and WO92/10544, Me3Pt(O O), where O O represents, for example, acetylacetonato or tropolonato ligands, known from WO95/25735 and (TMDVS)Pt-L, where L is generally a chromophoric ligand, known from WO95/25734. All these compounds display, if appropriate in combination with a suitable photoinitiator or photosensitization catalyst, catalytic activity in respect of the crosslinking of polysiloxanes at room temperature under the action of UV and/or visible light. Furthermore, these catalysts are used in the homogeneous phase and frequently display a higher activity and selectivity than comparable catalysts used heterogeneously.
A great process disadvantage of homogeneous catalysts is the difficulty of separating them from the product formed and/or solvent present and the difficult recovery associated therewith. A further disadvantage of hydrosilylation catalysts used homogeneously is the frequently quite short operating life caused by the formation of catalytically inactive species.
On the other hand, supported Pt catalysts often display lower activities or selectivities in hydrosilylation. However, they have, as (simple) heterogeneous catalysts, the advantage that they are substantially more robust and less sensitive, therefore have a longer operating life, and can also be separated and recovered more easily.
Platinum hydrosilylation catalysts immobilized on an inorganic support, frequently carbon, for example in the form of carbon black or activated carbon, are known from the prior art. Such supported catalysts are, for example, described in U.S. Pat. No. 2,637,738, DE 2815316 and DE 2012229 and also from P. Boudjouk, B. H. Han, Organometallics 1983, 2, 769-771.
Furthermore, γ-Al2O3 (U.S. Pat. No. 3,168,544, U.S. Pat. No. 2,970,150) has also frequently been used as support material for heterogeneous Pt catalysts.
EP 0904315 B1 describes heterogeneous, supported Pt catalysts on the supports carbon black, activated carbon, Al2O3, barium silicate and barium oxide for the hydrosilylation of C═C-functional epoxy compounds by means of SiH-functional polysiloxanes.
TiO2-supported Pt catalysts have generally been known for a long time, e.g. from DE 3340569 for the production of synthesis gas or H2, from EP 201670 for methanol conversion, from EP 730906 as oxidation catalysts, from DE 19905838 as hydrogenation catalysts or from WO 01041926 as hydrooxidation catalysts.
A process for preparing 3-functionalized propylsilanes by hydrosilylation of allyl chloride by means of SiH-functional silanes in the presence of a supported multielement catalyst in which one element is platinum supported on TiO2 has been described in DE 19857223. US 6177585 describes a process for the hydrosilylation of C═C-functional compounds by means of SiH-functional silanes in the presence of supported bimetallic catalysts comprising platinum on a support comprising titanium dioxide.
However, all supported platinum hydrosilylation catalysts known from the prior art display an activity which is too low, in particular for applications in which a particularly rapid reaction is required. Furthermore, the TiO2-supported platinum catalysts used hitherto for hydrosilylation reactions in prior art are mixed metal catalysts.
Particular catalysts comprising nanosize TiO2 and platinum or obtained from nanosize TiO2 by means of a sol-gel process are known from DE 4402873 A1, DE 2830231 A1 and DE 10025964 A1.
DE 4402873 A1 discloses compositions comprising platinum particles, polymers and nanosize titanium dioxide which are obtainable by a sol-gel process.
DE 2830231A1 discloses a platinum-containing supported catalyst whose support may be a porous gel of titanium dioxide, likewise obtained by a sol-gel process.
DE 10025964 A1 describes heterogeneous catalysts based on a nanosize magnetizable support material which may comprise, inter alia, titanium dioxide and platinum as catalytically active substance.
However, the prior art does not disclose any heterogeneous catalyst which consists exclusively of platinum applied to nanosize titanium dioxide.